Update time: Here is the latest reply from the DORS:-

Dear GlennTheBaker

Thank you for the note. I do apologize for all inconveniencies that you have to go through with your reservations, we were taken by surprise with the Renaissance leaving our group and we have to relocate hundreds of reservations as a result. I would like to point that the Marriott Executive Apartments is a very nice hotel in the city center, just five minutes walk from the Wenceslav Square. We can also offer the Courtyard by Marriott Flora or Courtyard by Marriott Airport for October. Unfortunately, the Prague Marriott Hotel is not available in this period. However, I have good news and can re-book the December reservation at the Prague Marriott Hotel if you wish.

I look forward to welcoming you in October to the Marriott Executive Apartments & offer my apologies in person. I would like to also offer you an upgrade to a One Bedroom Suite as well. I thank you very much in advance for your understanding of the situation.

We look forward to be of a service & thank you for giving us the chance to regain your trust.

I've replied again, saying that an Extended Stay apartment on the edge of town is no comparison to a 5* city centre full service hotel. The October reservation is for my mum (a sprightly 63 year old) so she can re-visit the scene of my marriage in May. The Exec Apartments are simply unsuitable for her 2-night stay. Earlier today, standard rooms were available - only Exec Level rooms and Junior Suites are showing now.

Just a question for someone with a bit of insider knowledge - is it possible for a hotel to leave the group without warning? Surely some notice period is needed?